<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .a {
            width: 100px;
            height: 100px;
            background-color: #f00;
            /* transition: 1s 1s ease; */

            /* all 默认所有属性都添加过渡效果 */
            /* transition: all 1s; */

            transition: width 1s, height 1s 1s;
        }

        .a:hover {
            width: 200px;
            height: 200px;
            background-color: #000;
        }
    </style>
</head>

<body>

    <div class="a"></div>

</body>

</html>